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MG MGB Technical - Ignition light glow at night--dynamo

There is no doubt this has been covered as a search of the archives leaves me with an overwhelming amount of info--almost too much so I'll ask again. Apologies.

In preperation for a 1500 mile journey Thursday I checked the charging system. Sitting in the driveway, in the dark, with only a few appliances on, say windscreen wipers and heater fan, the ignition light glows ever so slightly. If I continue to add load--lights, turn signals, brake lights--the glow of the light increases slightly more. If I turn everything electrical off, as in daytime, dry weather, there are no issues.

Here are the numbers:

1964 positive ground

Car off batteries read 12.8v
@3500 rpm, no electrics on 13.5v at battery
@3500 rpm, all appliances on 12.8v to 12.5v at battery

Upon connection of the two terminals of the dynamo @ 1000 rpm= 16volts
20 volts achieved at 1400rpm.

specific gravity of the batteries ranges between 1.255 and 1.265 at 65 degrees F. This is just out of the "fair zone" on my tester, but believe its within "good zone" spec of the Bentley manual.

I cleaned the control box points, and tighened all connections to the control box, solinoid and generator. I also cleaned all the battery terminals and even cut off the helmut heads and replaced with clamps. Still same indications.

Are there any other tests that I can do on the control box? He's my usual suspect!! Is the generator performing as it should? If I order parts by early afternoon tommorrow, I can get by Wednesday, so time is of the essence. One day, I will convert to an alternator perhaps, just not this week!

Thanks in advance

Paul
Paul Hanley

As an addition, I tested the field coils tonight. They were spot on a year ago at 6 ohms. Tonight my meter was jumping all over the place and attempted to settle around 12 ohms. That doesn't sound logical but thought I'd report it. My spare, rebuilt 2 years ago also read 6 ohms at rebuild but is now showing 3.5.
Oh my, I'm going to have to order a NEW, not lucas from Moss tommorrow. Say it isn't so. I'm kinda dead in the water here....

Good morning Mr. Hunt. The mission you are about to receive...


Paul Hanley

First , can you believe the meter, is there something else you can check it on , does it read steady when testing a light bulb for instance.
Next , thinking about how the control box works
At idle the cut out should be open , and then the charge (WL)bulb which runs from the battery to to the dynamo will glow as a small current flows to ground through the dynamo , when the voltage is high enough to allow a charge the cut out closes and shorts the bulb out,cut out is closed at 13v open at 10v
The dynamo is specified as 13 volts at 1585 rpm , for set up a 20 v meter is suggested , so yours sounds as though it's working OK. (this is with D and f shorted as you tested it )
When I had this problem the ign light reamained
on ,but very dim , at all revs , you had to cup your hand around it to see the glow in daylight .This was because resistance in the voltage control contact was preventing field current flowing .(this is NOT the cut out)
I had not considered what the ign light should do as you load up the battery with the cut out open , however I think it should get dimmer, as battery volts will drop , would be interested in any comments .
Secondly , it is worth checking the overtravel on the voltage control,this is different to the cut out ,
this buzzes in and out too fast to see when it is in circuit at 20C it should hold the dynamo to 14.7 to 15.3 volts . You conect ign and B take off the spade on the WL ( light) terminal and connect voltmeter between WL and ground. Thenrev to 3000 and measure volts
S Best

:o)

Paul - your dynamo test results seem a little low, the Workshop Manual indicates that 20v should be reached by 1000rpm, that is with the loom disconnected and the D linked to the F.

Your system voltage readings also seem low, with minimal load at 3000 rpm the book indicates about 15.2v at an ambient temperature of 50F, dropping to about 14.6v at an ambient of 105F.

Generally speaking the warning light acts as a pair of balance scales. With the ignition off and the engine stopped there is 0v each side and so no glow. With the ignition on and preparing to start there is battery voltage i.e. 12.8v on the white side of the lamp from the ignition switch and 0v on the brown/yellow side from the control box and hence it glows at full brightness. When the engine is running at above about 1000 rpm there should be about 15v both sides and so again the bulb does not glow. And just for completeness if you rev the engine and switch off the light will glow brightly immediately, then flicker and die as the engine spins down. This is because the dynamo is still generating 15v on the brown/yellow but now there is virtually a ground on the white through the relatively low resistance - as far as the warning light is concerned - of the coil.

If the warning light glows slightly and increasingly as more electrical load is applied to the system then using a meter you will find that both sides of the bulb will have voltage in the range 12v to 15v but one side will be higher than the other. Usually this is because of slightly high-resistance connections somewhere between the D terminal of the dynamo, through the low impedance windings on the cut-out and voltage regulator relays and the cut-out contact to the B terminals in the control box, and from the B terminal on the control box to the ignition switch and through that to the white wire connected to the warning lamp. If you measure the voltage on the D, WL and B terminals of the control box, and the brown and white of the ignition switch, and the brown at the fusebox and main lighting switch (headlights on) and compare them, you should be able to find if you have any such bad connections. However because the brown that feeds the warning light and that which feeds the lights part company at the control box the problem would appear to be on the control box spades, connectors, or withing the box. In practice there will always be a small voltage difference across the warniong light but it should not be enough to make it glow even dimly.
Paul Hunt

Paul , my manual , printed in 1967 , is very coy about dynamo output when testing with field shorted to output, it only says do not allow it to rise above 20 v , and there is no need to run it beyond 1000 rpm. There is also the comment that you need only a 20v meter .You are refered to general data, which is where the 13v at 1585rpm "cut in" voltage whatever that means is mentioned .I have not carried out these tests for many years but seem to recall that mine achieved something like the 13v at 1500 rpm. The manual helpfully gives you anticipated voltages if
the field coil is faulty , 1 to 1.5 v , and if the armature has a problem when 4 to 5 v may be seen.The implication is that seeing 13 volts means the unit is functional. As current or voltage rise in service either the voltage control or current control coil switch in the field resistor and reduce output. The cut out does short the warning light as it connects B to D via the heavy gauge windings on the current regulator and the cut out coil , thus it goes out once the battery is charging .I did draw out the full theoretical circuit when I had my problem , and will try to find it , scan it and post it somewhere (perhaps I could e mail it to you ) as I would welcome a chance to talk about it .Testing the overcurrent coil would be interesting , 22 amps at 12 volts is nearly 1/3 kilowatt , if I was going to make up a test jig I would use headlight bulbs in parallel I think .
I do recall that my box was set up from the factory to cut in as soon as possible , and to give the max dynamo volts to give best charging , I was impressed as this gave the best possible performance with the kit extant until the alternator solved all these problems.
S Best

Thanks for the input so far Paul & Mr. Best. As I'm electricaly challanged, I'm still trying to understand what you both are telling me. Will run the checks for faulty connections. I did order a new generator today along with a few voltage regulators. The gross difference in the field coil ohm readings concernd me enough that the generator might be starting to give up the ghost and those darn regulators have such a bad reputation these days. One item I forgot to mention was the dramatic increase in turn signal tempo at higher rpms vs idle along with the brightening of the headlights

BTW, those that might be interested, the field coils on the generator are one of the few parts on our cars that are not rebuildable, so don't toss your old generators because of other issues.

Since I am waiting for parts, its off to the barn for the "tighten that spade an bullit connection" game as suggested. We're resolved here to get to Chattanooga one way or another. Either with the B--fixed or daylight driving only, take the A with the boot full of perhaps a change of shorts, and a quart of oil and be frozen to the seat by Roanoke. Don't thik the wife will go for the A option!

Cheers

Paul
Paul Hanley

Sorry to blind you with science , I suppose you either love or loath this sort of thing .
It may be worth checking the earth on the regulator box, the voltage control coil is earthed via a swamp resistor which reduces the effect of the change in resistance with temperature of the copper coil . If this or the earth have gone high impedance then the regulation of voltage against RPM will not work , causing the bright headlamps/fast indiators you are seeing .
It is quite normal for the headlamps to brighten slightly as you rev from idle , when the cut out goes in they will jump from the 12v of the loaded battery to the 13 v the cutout is set to .Have a great trip !
S Best

Some good news. Have been trickle charging all day and have batteries clearly in the green. I took my dremel tool with small wire brush and mirror shined all the regulator and generator connections. The small F connection was particularly dirty. I also polished the paint off in a little descrete corner of the generator casing and pleasantly wittnessed the field coil test return to 6 ohms. Did the same with the spare and achieved 5.6 ohms.

As for the main issue of the ignition light it's still a factor but muched reduced. I can run with headlights and one other appliance if necessary before the glowing returns. Even with everything running and lamp a glow, I'm still getting 13 volts to the batteries. Since were not talking aircraft mechanics here, I deeming this machine roadworthy. If my Moss order shows up tommorrow, I'm replacing the regulator as its still my vote as the culprit. As such, I wasn't about to start pulling apart the headlight loom and bullit connections. My luck, and some tips will break off of the wires and I'll really be stuck. Theres little doubt that some will break as I made sure the connectors were tight when I assembled. But it does need to be done as this car was built without vasaline or dielectric grease. Sorry, didn't know about all the great help available then and it wasn't mentioned in the Haynes manual!

I'll post tommorrow and let you know the results of the regulator swap but for now, the battery cover and carpet are back in. Hope for the best. Any other thoughts are certainly appreciated.

Back to waxing and polishing--already got another coat on tonight! "Pardon me boy....

Paul
Paul Hanley

Fixed!!

Twas the white wire at the ignition switch. Spade connection was loose. Tightened and presto, no more issue. Looks like I'm returning a few parts to Moss.

In all candor, I can run everything on the car, in complete darkness and see absolutly zero through the red jewel lens. If I take the lamp fixture out of the tachometer however, there is an ever slight orange color on the very tip of the filament, barely detectable to the eye. It only lights when I have everthing on, including brakes. I'm chalking it up to the loose rivet holding the spade to the ignition switch itself. I attempted to tighen the rivet with a small punch and chucked off a piece of plastic from the switch. Decided it was time to stop fixing before I broke something and just drive it.

Thanks all and special thanks to John Twist of University Motors Ltd., who once again so effortlessly plays the beacon at the end of a stormy runway.

We're off the see the lizzard...er..Dragon.

Cheers,

Paul

Paul Hanley

Paul yesterday I saw a NOS generator on eBay, dunno if it is still there.
Carl W French

re: We're off the see the lizzard...er..Dragon.


paul....

Just remember...Dramamine 50mg or
Meclizine 25mg at least 20 minutes before that first turn!

have fun, you lucky b*@#^rd!

rick
rick ingram

It would be permissable for a very slight voltage to be on the lamp at full load, I have not measured the resistance in the box between D and B , you would have to disconnect the battery and then push the cut out in
( NEVER TOUCH THE CUT OUT WITH THE BATTERY AND DYNAMO
IN CIRCUIT YOU WILL DESTROY BOTH)and there would be a ? about whether you had the right contact pressure , and resisatence .However say you were drawing the full 20A then 20 milli ohm gives you 0.4v which may just be visible at the highest impedance point on the filament. You could put your voltmeter between D and B on the control box and measure this voltage.
A resistance in the feed to the coil would cause a volt drop at the battery end of the WL and current would flow through the bulb from the dynamo as a result. You could see this voltage with the meter between B on the contol box ,and SW on the ign coil .
Looking at the circuit I believe that poor contacts between the lamp and WL would not cause this , but may prevent the lamp from glowing .
Thanks for posting this , it's been very interesting , I will take some measurements on my car and post the results .
S Best

Although the cut-out relay operates at about 13v the system voltage continues to rise rapidly as revs increase to about 15v in normal use, unless there is a large drain on the output. There will always be a tiny bit of voltage across the warning light in normal use but if you can't see this through the red lens then you can ignore what you can see on the bare bulb.

Not sure about destroying the battery and the dynamo if you touch the cut-out relay. If the engine isn't revving enough for the cut-out to be operated then manually closing it applies battery current to the dynamo which just tries to make it motor faster than the engine is already spinning it. Whilst this does take some charge out of the battery it is nowhere near the current drawn by the starter, for example. If the engine were revving at 2000 or 3000 rpm or more and you pulled the cut-out relay contact open or insulated it, or some fault effectively caused that to happen, then you could damage the dynamo from over-voltage (i.e. allowing it to develop more than 20v at its output) but not harm the battery.

The bulb glows because of resistance in the control box itself or in the brown and white wires running from the B terminal on the control box through the ignition switch to where the white for the warning light is connected, and where Paul Hanley found his loose connection falls right into that category. Any bad connections in the white between where the warning light is tapped in and the coil +ve will not cause the warning light to glow, although it will affect spark performance of course. Likewise bad connections from the D terminal back to the dynamo will not cause the light to glow, just limit maximum current the dynamo can deliver. Of course if this bad connection is bad enough it may not allow enough voltage to be developed to operate the cut-out relay in which case the warning light will be glowing brightly.
Paul Hunt

Hi Paul ( I mean Paul Hunt) you are of coures right about where resistance will cause the bulb to glow , and whether it matters when the cut out is working properly .Between the battery and where the ignition
loads are taken will cause the light to glow ,
between WL on the control box and the point where the other ign loads are taken ( the switch) will have no effect. I would like to return to the effect of a poor connection between the dynamo output and D on the control box later.
They key thing to understand here is how the cut out works , it's a lot cleverer than you might think , it has to be , a simple 13v coil would switch on and stay latched on from battery voltage .. It is not polarised , but works sequentially .It has a voltage high resistance parallel and a series low resistance current coil , when you start the engine the cut out is open so only the voltage coil is in circuit. As volts rise to 13 this pulls in and then charging current starts to flow in the series coil , this sets up an additional field which is in the same direction as the voltage coil field . If the engine slows down to where the dynamo volts are less than the battery votls then the direction of current flow in the current coil changes, and now opposes the field due to the voltage coil , this is how you get the hysterisis and also how the cut out is made to open, These fields are set such that by 9 V the cut out is forced open , and the WL will come on. This is how you see the bulb
glow and vary with engine RPM at idle .Now if you were to push the cut out in with the engine stopped , there would be no curent flowing in the voltage coil since it is shorted by the dynamo alternator , which cannot motor to produce a back EMF as it is braked by the engine. Since the cut out is soft iron based magnetic circuit the current flowing in the current coil will hold in the cut out and cause destructive currents to flow , either the dynamo , or the control box would be destroyed .
If however the cut out failed open circuit while the car was running, of course WL would come on but the voltage control coil has sufficient authority via the field resistor to hold the dynamo at 14,5 to 15 volts .The voltage driving WL would be the 3 v or so that the dynamo exceeds the battery by . This is valuable as it will signal to the driver that he has a charging circuit problem . If there was a resistance between dynamo output and D then this would be signalled by the WL coming on as loads were swithched in and the dynamo could not keep up , throwing load onto the battery .
Very happy to mail you the full theoretical circuit as this is so intereting .
S Best

Well, by practical experience I have to disagree. I have manually operated the cut-out and all it does is to cause battery voltage to spin (or try to spin if the fan belt is still fitted) the dynamo. The current that flows is no more destructive than if the battery were to be connected directly to the dynamo and not via the control box. This shouldn't be surprising as a dynamo is simply a motor and vice-versa. Even if the dynamo is restrained by the fan-belt there are no destructive currents, and if the motor is allowed to spin freely with battery voltage applied the current is even less. With the cut-out manually operated current flows through the voltage winding of the cut-out relay in series with the field winding, without this the dynamo wouldn't try to motor. Neither does the cut-out relay latch, but releases under spring pressure. As I say this is from personal experience on my own cars.
Paul Hunt

Firstly, let me apologise for a failure in my proof reading in my post of 5th Nov. Please read, "since it is shorted by the dynamo armature " not "dynamo alternator “. This tripped off my keyboard unbidden.
Re your experiences with manually operating the cut out. This is strange, I have seen the cut out latch, and coils start to smoke on an MGB, and heard of other people destroying regulator boxes (on T Types). In all cases this was by manually operating the cutout. So I have to stand by my advice. In the case of the MGB I manually forced open the latched cut out contacts and order was restored. This was on a car that had ign off! Certainly in this case battery voltage is available to the field winding, (as you say) although if more than 20A flows the field resistor will be switched into circuit via the action of the over current coil. However even in this condition there would be plenty of field to allow the dynamo to motor without the fanbelt in place. I have no intention of experimenting with manual actuation of the cut out on my own car and there are no occasions when with the control box working as designed the battery will connect to the dynamo when the ign is off. If someone has spare box and a mechanically dead but intact electrically dynamo then I would be keen to have a look at this on the bench using a lab power supply.
S Best

Anyone who has been following this thread, please go here and read

http://www.mgb.bc.ca/electrical/generator.html
S Best

This thread was discussed between 02/11/2004 and 10/11/2004

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